The operation of nuclear reactors, especially those utilized for electric power generation, generally employs substantial amounts of water for cooling and fuel handling. This water acquires a significant amount of radioactive dissolved and dispersed solids from activation of impurities and corrosion products in the reactor. Contaminated borated waste water is also obtained from reactor letdown. Condensate polisher regenerant solutions, high conductivity floor drain streams and waste water from contaminated laundry are also sources of radioactive aqueous waste streams. These streams, and others, are collectively often referred to as radwaste streams.
Because of the substantial volume of the radwaste streams they cannot be disposed of without being substantially reduced in volume. The radwaste streams are accordingly collected, sometimes stored temporarily, and then processed to a small volume of greatly increased solids content generally called bottoms. The concentrated waste has a significant radioactive level so that it must be properly disposed of. The concentrated or waste or bottoms is, for example, solidified with a binding agent, put in strong containers, and then buried underground.
Radwaste volume reduction, or concentration, systems generally employ a steam heated forced circulation evaporator, a feed tank and a conduit for supplying low radioactive aqueous waste from the feed tank to the evaporator. During start-up of the system, the feed stream from the feed tank is supplied to the evaporator and concentrated without removal of bottoms until the solids level reaches the desired concentration. Since the feed stream rate can be as little as one-hundredth the rate at which the liquor is recirculated it is easy to maintain solids equilibrium in the evaporator. The water from the feed stream is, of course, evaporated except for the small amount which is withdrawn with the bottoms.
As the concentration of solids or bottoms increases in the evaporator, a level will be reached at which removal of bottoms becomes prudent, if not essential, to maintain evaporator efficiency. Thus, an evaporator may have a capacity of 1,500 gals. of liquor. When the concentration of solids reaches a predetermined level, such as 25% solids by weight, the liquor level is lowered, as for example 2 feet, by removal of 400 to 600 gallons of bottoms. This volume of liquor is then replaced from the feed stream, resulting in dilution of the liquor in the evaporator. Evaporation proceeds, with continuous addition of dilute radioactive waste, until the liquor again reaches the predetermined solids concentration. Bottoms can then be removed as described.
In the past, conventional piping and valves have been used to periodically drain bottoms from the evaporator and feed it to a bottoms tank for further handling and processing. The high solids content of the bottoms resulted in plugging of pipes and valves, causing shut downs and necessitating repairs. Because of the high radioactivity of the bottoms, repair work could be done remotely with suitable protection against radioactive exposure of the repairmen. Furthermore, the prior systems of removing bottoms provided no means of fully emptying the drain pipes so that the radioactive residue remaining in the pipes made it unsafe for workers to come into close proximity of the equipment. Isolation of the highly radioactive bottoms withdrawal areas of the plant was thus required, thereby limiting plant access, inspection and repair and providing an additional unsafe area.